Heat exchangers for heating water are used for sanitary washing apparatuses that wash the private parts of the human bodies, clothes washing apparatuses that wash clothes, and dish washing apparatuses that wash dishes (see Patent Document 1).
FIG. 48 is a schematic sectional view of a conventional heat exchanger. As shown in FIG. 48, the heat exchanger has a double pipe structure comprising a cylindrical base material pipe 801 and an outer cylinder 802. A heater 803 is provided outside the base material pipe 801. A spiral core 805 is inserted into an inner hole 804 of the base material pipe 801. Washing water flows along a screw thread 806 on the spiral core 805 in the inner hole 804 of the base material pipe 801. At this time, heat exchange between the heater 803 and water causes warm water to be generated.
In the conventional heat exchanger, however, water is heated to approximately 40° C. by the heater 803, so that a scale such as a calcium component contained in water is deposited on an inner surface of the base material pipe 801 and a surface of the spiral core 805 to adhere thereto. This results in reduced heat exchange efficiency. When the heat exchanger is employed for a long time period, the scale closes a flow path, so that water does not flow. Thus, a boil-dry state occurs. Similarly, other impurities such as a water stain and dust are also deposited on the inner surface of the base material pipe 801 and the surface of the spiral core 805 to adhere thereto. Consequently, the life of the heat exchanger is shortened.
Since the heater 803 is provided on an outer surface of the base material pipe 801, an outer cylinder 802 for thermally insulating and surrounding the heater 803 is required. Therefore, it is difficult to miniaturize the heat exchanger.
Furthermore, heat generated by the heater 803 provided on the outer surface of the base material pipe 801 escapes out of the base material pipe 801, resulting in poor heat exchange efficiency.
Since the spiral core 805 is inserted into and held in the inner hole 804, the spiral core 805 comes into contact with the inner surface of the base material pipe 801 heated by the heater 803. Therefore, the spiral core 805 must be formed of a material having high heat resistance. Consequently, a material for the spiral core 805 is limited, which makes it difficult to make the heat exchanger lightweight.
Such a conventional heat exchanger is used for a sanitary washing apparatus that washes the private parts of the human body, for example. However, impurities such as a scale are deposited on the conventional heat exchanger to adhere thereto due to long-term use. When a large number of fractions of the impurities that have adhered to the heat exchanger are discharged from the heat exchanger, a washing nozzle is clogged, so that washing water cannot be sprayed. As a result, the life of the sanitary washing apparatus is shortened.
Since the conventional heat exchanger is difficult to miniaturize, a sanitary washing apparatus using the heat exchanger is also difficult to miniaturize.    [Patent Document 1] JP 2001-279786 A